Membranes play a crucial role in almost all cellular phenomena, yet we are only beginning to understand the molecular organization of these structures and functions which they carry out. The lipid components of membranes have been well characterized and the organization of lipid in membranes has also been well studied. In recent years similar studies of the proteins of membranes have yield much useful information about membrane structure and function. The study of the physical, chemical and enzymological properties of the enzymes of phospholipid biosynthesis should extend this knowledge even further. These enzymes are for the most part membrane associated, they carry out the synthesis of the lipid component of membranes and therefore are involved in membrane biogenesis and assembly and cell growth. Through the study of the purified proteins first in membrane model systems and then in reconstituted membrane systems we should learn more about cell growth and its control.